An automated method for the rapid measurement of diffusion coefficients has been developed. Using this method, the diffusion coefficients of each of several dilute tracer proteins have been measured as a function of the concentrations of each of several unlabeled background proteins at concentrations of up to 200 g/l. A microcomputer program for simulation of sedimentation in a centrifuge has been generalized to treat substantial deviations from thermodynamic ideality, multiple solutes, and self- associating solutes with arbitrarily specified association and dissoclation rate constants. Sedimentation equilibrium experiments conducted on three different proteins over a broad range of concentrations have revealed that two to them exhibit previously undetected self- association at high total protein concentration. The competition of labeled and unlabeled ligands for common acceptors has been analyzed for the case of multiple classes of acceptors, cooperative binding, and multivalent ligands. Extensive data on the indirect interaction between the neurotransmitter acetylcholine, which binds to muscarinic receptors, and batrachotoxin, which binds to sodium channels, have been analyzed quantitatively using a model postulating that each of the ligands modulates the affinity of its respective acceptor species for limited amounts of two different effector species, tentatively identified as different forms of the G-protein.